Fix defcustom :type of mu4e-headers-leave-behavior

[tsdh]

The const values of the choice had the wrong order (const :tag <value> <text>).
The correct order is (const :tag <text> <value>).

This resulted in the following warning when loading mu4e:

⛔ Warning (emacs): Value ‘apply’ does not match type (choice (const :tag ask ask user whether to ignore marks) (const :tag apply apply marks without asking) (const :tag ignore ignore marks without asking))

[djcb]

Merged; thanks!